In extruding thermoplastics pipes, plastics granulate is melted in an extruder to a homogeneous melt. Heat required for melting of the plastic is produced in the beginning of the process by means of electrical heating elements on the outer surface of the extruder cylinder. In addition, friction heat is produced during the extrusion by interaction between extruder screw, cylinder and the plastics granules. After the process has reached a stable state, the granules will melt mainly by friction heat. The molten mass is pressed through a heated tool to form a pipe profile of predetermined geometry. This molten pipe profile is formed to its final geometry by cooling it in a calibrator (sleeve) having a predetermined inner diameter determining the final diameters of the extruded pipe. This is carried out using vacuum or pressure calibration.
In vacuum calibration the pipe is sucked against a perforated surface of the calibrator by means of vacuum, which calibrator is arranged inside a cooling tank at a pressure lower than the atmospheric pressure prevailing also inside the pipe.
In pressure calibration an overpressure is created inside the pipe to press the pipe against the surface of the calibrator inside a cooling tank with normal pressure. This takes place by closing the interior of the pipe with a plug at the end of the pipe as well as by pressuring the closed space by means of pressurized air lead through the extruding tool.
In both calibration versions the pipe is typically cooled by spraying the outer surface of the pipe with cooling water in one or several calibration tanks. The pipe has to be cooled sufficiently in order to be able to be handled along the process line in marking, drawing and cutting devices. A certain stability is also required to enable particularly large-sized, thick-walled pipes to withstand their own weight without being deformed during their handling and storing directly after the extrusion.
The production rate of the aforesaid thermoplastics pipes is often determined by the cooling capacity of the line. A part of the heat is transferred inwards through the inner surface of the pipe, but a major part of the melting heat is transferred by water cooling at the outer surface.
Cooling concepts used in existing extrusion lines encounter a number of potential problems. In manufacturing thick-walled pipes the portion of plastics still in molten condition may be so large that its weight exceeds the strength of molten plastics directly after the die and inside the calibrator. Due to this, molten plastics flows from the upper side of the pipe towards its lower part, thereby resulting in non-uniform wall thickness. The pipe wall will become thinner in the upper part of the pipe than at the sides and in the lower part of the pipe. This can be compensated by unevenly centering the slot in the extruding tool, through which the molten plastics flows. The slot is thus arranged to be largest on the top, thereby obtaining a final product with a relatively uniform wall thickness distribution. Since the product tolerances define minimum and maximum wall thicknesses as well as maximum values of faulty eccentric running, the aforesaid method leads to material losses and reject.
A further problem arising during the extrusion is that thermoplastics material oxidizes when the inner surface of the pipe for a long time is exposed to hot oxygen-rich atmosphere. This results in impaired mechanical properties, such as pressure strength in pressure pipes designed e.g. for water or gas pipe lines. A possibility to counteract this is to introduce, for example, an inert gas into the pipe, which, on the other hand, leads to increased production costs.
Cooling of the pipe only from the outside affects also the inner stresses in the pipe wall, resulting in a stress profile, which leads, e.g. when the pipe is cut, to lacing of the pipe end.
A method and a device according to the preamble is previously known from US patent specification U.S. Pat. No. 6,418,732, wherein solid and gaseous carbon dioxide is used as cooling medium. This leads to a too low temperature of the cooling medium resulting in considerable condensing water problems inside the thermoplastics pipe which may cause serious defects in the pipe wall. Further the process according to this previously known process is relatively cost consuming.